Please put your system specs in here - seriously, it helps more than you think.
software voltage readings can be wildly inaccurate, those BIOS readings seem reasonable - I don't think that's the issue
I have never seen a GPU downclock because of voltages, they either work or crash - its clocking down because its not being asked to do anything, or because it's triggered a thermal throttle
Perfcap PWR is BIOS or software limited TDP, not an issue with input power - MSI afterburner or GFE's over/underclocking can do this, as an example.
This sounds like some sort of software issue, or a throttling issue
For examples:
If your CPU is overheating/throttled, the GPU wont clock up because it has nothing to do
Or, you could have something set wrong in windows somewhere, or in the NVidia control panel
Can you post a screenshot of Hwinfo showing all the sensors (Fullscreen it, show all the readings) before a game and one screenshot while a game is running minimized or windowed?
Can you confirm your BIOS is on default settings, other than enabling XMP or altering fan speeds?
Can you reinstall your Nvidia driver with the 'clean install' option and confirm you have no runnings programs in your windows tray area?
Can you confirm windows is on the default balanced or high performance plans?